The invention relates to controlling a protective air flow rate of a low-pressure turbine in an aircraft reactor, and more particularly detecting an increase in the rotor speed of the low-pressure turbine.
Depending on the flight phase of an aircraft, the evolution of the speed of a turbojet engine causes a deformation in the vanes of the low-pressure turbine as well as the casing of that same turbine. These deformations are due on the one hand to the increase or decrease in the temperature of the low-pressure turbine, and on the other hand to the effect of the centrifugal force exerted on the vanes of the rotor of the turbine.
This phenomenon results in modifying, during a flight of the aircraft, the distance between the apex of the vanes and the surface of the casing. When the play between the apex of the vanes of the turbine and the casing increases, part of the air suctioned in the casing no longer passes into the turbine. The performance of the engine is then decreased, and the consumption of the turbojet engine increases to obtain the same speed.
It is therefore necessary to cool the casing of the low-pressure turbine more or less in order to continuously minimize the distance separating the apex of the vanes and the casing of the low-pressure turbine.
In order to cool low-pressure turbine, cold air is extracted from the secondary flow taken at the fan and/or the compressor of the turbine engine, in order to be conveyed via channels to the outer surface of the low-pressure turbine.
Along these channels, an air valve with a regulated position, referred to as LPTACC (Low Pressure Turbine Active Clearance Control), makes it possible to regulate the air flow rate to be sent onto the turbine according to the setpoint from the electronic engine control (EEC) unit.
Given that the deformations of the casing are only due to the thermal expansion, while the vanes undergo deformations due both to the thermal expansion and the centrifugal force, the elongation of the vanes is generally greater than the radial deformation of the casing.
The vanes deforming more than the casing at the same rotation speed and the same temperature, the apex of the vanes risks wearing the abradable coating of the casing and thus causing permanent incurable play without repair between the apex of the vanes and the casing in which the vanes move.
During a cruising phase of a flight, the thrust from an engine may suddenly increase for several reasons, for example a gust of wind or a change in altitude ordered by air traffic control. The engine speed then increases from a cruising phase level to a step-climb phase level.
The sudden increase in the rotor speed of the turbine causes sudden deformations of the vanes due to the thermal expansion and the centrifugal force.
However, the aircraft being in a cruising phase, the cooling air flow rate is optimized to reduce deformations of the casing. As a result, the sudden increase in the rotor speed of the turbine during the cruising phase causes a quicker and more substantial deformation of the vanes, due to the deformations generated by the centrifugal force, than the deformation of the casing.
This difference in deformation amplitude then causes a significant risk of wear of the abradable coating.
The known systems for regulating the cooling air flow rate of the low-pressure turbines of aircraft have no logic for detecting the different flight phases. Consequently, there is a significant risk of wear of the abradable coating, in particular upon each sudden increase of the rotor speed of the low-pressure turbine during the cruising phase.